The invention relates generally to the display of digital images. More specifically, the invention relates to an apparatus and method of performing dithering to enhance the quality of displayed images.
A computer monitor having 24-bit color capability can display photo-realistic images. Such high quality can be achieved in part because the 24-bit color capability provides a choice of 16.7 million discrete colors for each individual pixel of the image being displayed.
A conventional inkjet printer, on the other hand, does not have a choice of 16.7 million colors for printing an image. The conventional inkjet printer deposits dots of colored ink on a sheet of paper. Within the space of each dot, the conventional inkjet printer might deposit one of eight solid colors: cyan, yellow, magenta, black, white, red, green and blue. Thus, the conventional inkjet printer has a choice of eight printable colors.
The inkjet printer might use a digital halftoning method to create a perception that it is printing with a wider range of colors than it actually is. When printing a solid color patch, the inkjet printer can place dots of different colors in a high frequency pattern. When viewing the high frequency pattern, the human visual system "applies" a low-pass filter and perceives only the average color of the dots in the pattern. Thus, a dot pattern including black and white dots appears as a shade of gray. The shade of gray depends upon the relative quantities of black and white dots in the pattern.
Monochrome dithering is a halftoning method in which the gray values of a monochrome image are compared against an array of thresholds. Ink dots are deposited in those locations where the pixel values are less than or equal to the threshold values. Standard (Cartesian) dithering is an overlay of three monochrome dithering processes, each of which is applied separately to a single color plane. Thus, a halftone color is a Cartesian product of three halftoned monochrome planes corresponding to color components. When printing a halftone color, potentially all eight colors are used in a solid color patch.
To produce a good halftone color, the color dots are placed such that their placement pattern is visually unnoticeable. Additionally, the dot colors are selected such that the local average color of the dots is the desired color of the color patch.
However, there are problems associated with halftoning methods, especially at printing resolutions of up to 600 dots per inch ("dpi"). Because inkjet dot patterns at 600 dpi can be resolved by the human eye, the dots might be perceived as artifacts and other types of noise. The noise might degrade the quality of the printed image and result in a grainy image.
Efforts have been made to improve print quality by increasing the color choices at each dot. This has been achieved by controlling the amount of ink deposited at each dot. Varying the amount of ink can control the intensity at each dot. The HP DeskJet 850C inkjet printer, available from the Hewlett-Packard Company, can vary the amount of ink deposited at each dot to print up to 64 different color and grayscale choices at each dot. Smoother dither patterns and halftones result in a less grainy image.
Other efforts have been made to minimize the noise in the image in order to produce a smoother dither patterns and halftones. For example, efforts have been directed towards determining optimal patterns for placing the dots. Efforts have also been directed towards determining an optimal number of dots that should be used for each color.